German police detained six Syrian nationals Tuesday who are suspected of planning an attack against a Christmas market in the city of Essen.

The suspects were detained in three different cities after an operation involving around 500 police officers. The men, aged between 20 and 28, are believed to be Islamic State members.

“The accused are also suspected of preparing an attack on a public target in Germany using weapons or explosives,” Christian Hartwig, a spokesman for the Frankfurt prosecutor’s office, said in a statement.

Policed were tipped off about the suspects from other Syrian refugees. The men were allegedly fighting for ISIS in Syria before arriving in Germany.

Authorities have boosted security at Christmas markets around the country after a Tunisian migrant killed 12 people in a truck attack in Berlin last year.

Prosecutors have opened more than 900 cases so far this year, compared to 240 throughout 2016. Just 80 cases related to terrorism reached the courts in 2013.

Germany’s federal police (BKA) estimates 705 Islamist extremists are willing to carry out terror attacks, up from 600 during an estimate in February. Germany’s domestic intelligence agency (BfV) recently said around 24,400 Islamists are active in the country but most of them don’t pose an immediate terror threat.

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